Project Summary The Henipavirus genus of RNA viruses in the family Paramyxoviridae contains five established species. Henipaviruses are found naturally in bats in Australia and Asia and more recently have been found in Africa, and they have a wide host range. These zoonotic pathogens can cause severe illness and death in domestic animals and humans. The prophylactic and therapeutic options for Hendra and Nipah virus infections in man are limited. Anti-Hendra/Nipah human mAbs are expected to be valuable antiviral therapeutics as countermeasures to Hendra and Nipah virus disease in humans. Here, we will isolate panels of naturally occurring human monoclonal antibodies (mAbs) that bind cross-reactively to both Hendra and Nipah virus F or G proteins and neutralize both viruses. In preliminary experiments, we have isolated some mAbs that exhibit very high potency in neutralization assays, suggesting they have high potential as prophylactic and therapeutic molecules for humans. We propose here a series of aims that will contribute significantly to the development and characterization of such human mAbs reactive to the F and G glycoproteins of Hendra and Nipah virus in preparation for clinical studies. The work will identify and fully characterize a panel of highly promising antibodies with the goal of identifying and selecting lead compounds and advancing their preclinical development. The work is organized in two major Specific Aims: Aim 1) Isolation of human mAbs from patients previously infected with henipavirus or exposed to henipavirus vaccine antigens. In this Aim, human mAbs will be identified that recognize epitopes that are conserved across henipaviruses and neutralize those viruses at low concentration. Blood cells from a subject exposed to Hendra virus equine vaccine will be screened for virus specific antibodies. These cells will be converted to stable human hybridoma cell lines and subjected to high- throughput screening to identify Abs that bind to Hendra and Nipah G proteins and functionally inhibit virus replication. Aim 2: Develop Abs for the treatment of henipavirus infections. Antibodies identified in Aim 1 will be tested for their broad recognition of conserved epitopes across all henipaviruses and for their ability to neutralize in culture. Prioritized antibodies then will be tested for therapeutic efficacy in multiple animal models of infection including nonhuman primates. The leads will be selected, and CHO cell lines will be made by Mapp Biopharmaceutical for Ab production, in preparation for cGMP manufacture and IND planning. The work promises to yield a best-in-class antibody preparation for broad and potent activity against henipaviruses that can be used to treat or prevent human henipavirus infections.